Energy efficiency of separated flow control by a nanosecond-pulse-driven plasma actuator (ns-DBDPA) was evaluated via wind tunnel experiments with a flow velocity of 40 m/s under atmospheric pressure. The dependence of the lift and drag coefficient on the different voltage amplitude shows that the optimal operating condition of the ns-DBDPA is estimated not by the sum of the discharge energy per unit time (discharge power) but by the discharge energy per single pulse. The results of the particle image velocimetry (PIV) show that the two vortices are shed by the pulse discharge from the leading edge of the airfoil where the ns-DBDPA is placed. Schlieren images show that the trajectories of the heated-zone produced by the discharge are equivalent to those of two vortices. These results indicate that the change in gas density caused by inputting the discharge energy to the air induces the formation of two vortices, thereby resulting in flow control.